The present invention relates to electrical circuits, and more particularly concerns a high-speed analog/digital converter (ADC) for converting a differential input signal into digital form.
In many applications, differential signals are preferred to single-ended analog signals. Differential signals have better noise rejection for low-level signals, and less pickup in high-impedance circuits, for example.
Conversion of differential signals to digital form, however, presents unique problems. Differential signals usually float with respect to any fixed reference voltage such as ground potential or the set of fixed voltages which must be used to compare against the analog signal. The average value of the input signal may vary from a nominal zero value, thus moving the entire signal up and down unpredictably against the fixed reference voltages.
The prior art in this general area has never faced these problems directly. U.S. Pat. No. 4,578,715 to Yamaguchi deals with a single-ended, ground-referenced analog signal so that its peak value always lies within the range of the reference voltages in an ADC. U.S. Pat. No. 4,110,745 to Ninomiya clamps the lowest voltage of an analog input in an ADC, using a known point in the signal. U.S. Pat. No. 3,544,779 to Farrow senses the average value of a single-ended analog input, but uses this average only to vary the various slicing levels of an ADC.